1. Field of Invention
The current invention relates to systems and methods of producing emulsions, and more particularly to systems and methods of producing emulsions that include recycling surfactant.
2. Discussion of Related Art
Extreme emulsification can be used to produce oil-in-water nanoemulsions with droplets that have radii, a, as small as about a˜15 nm, yet typically a significant quantity of surfactant must be used to reach such small sizes (T. G. Mason, J. N. Wilking, K. Meleson, C. B. Chang, and S. M. Graves, Nanoemulsions: Formation, Structure, and Physical Properties, J. Phys.: Condens. Matter 18 R635-R666 (2006), the entire content of which is incorporated herein by reference). For lower surfactant concentrations that are more economical, droplet sizes are typically larger, in the range of a>40 nm. To reach sub-40 nm droplets using certain surfactants, it is typically necessary to use significant quantities of surfactant (even exceeding 10 mass percent for some surfactants) in the emulsion composition. A high concentration of surfactant in the continuous phase can serve multiple purposes that can be desirable in the production of extremely small droplets when fabricating nanoemulsions through droplet rupturing and breakup. A larger quantity of surfactant typically lowers interfacial tension between the continuous phase (i.e. ‘base’ liquid) and the dispersed phase (i.e. droplet material); it also raises the viscosity of the continuous phase, which can promote droplet rupturing to smaller sizes; and it can also more strongly inhibit flow-induced coalescence of droplets during extreme emulsification by providing a stronger short-range repulsive interaction between droplet interfaces. In making the smallest nanodroplets, it would be useful to have a method that can recover, re-use, and re-cycle at least a portion of the surfactant without causing coalescence of the droplets, since a high surfactant concentration may not even be desirable in a final product. The higher surfactant concentration can provide a route to a smaller droplet sizes at lower flow rates. After droplets have been ruptured down to nanoscale sizes through an emulsification process, a much lower concentration of surfactant (e.g. 1% or less by mass) is typically satisfactory to inhibit coalescence of the droplets, even at high enough droplet volume fractions where the nanoemulsion becomes a soft elastic solid. Therefore, there remains a need for improved systems and methods of producing emulsions.